A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, polarized light; the common types of polarizers are circular polarizers. Polarizers are used in many optical techniques and instruments, polarizing filters find applications in photography and LCD technology. Polarizers can be made for other types of electromagnetic waves besides light, such as radio waves, X-rays. Linear polarizers can be divided into two general categories: absorptive polarizers, where the unwanted polarization states are absorbed by the device, beam-splitting polarizers, where the unpolarized beam is split into two beams with opposite polarization states. Polarizers which maintain the same axes of polarization with varying angles of incidence are called Cartesian polarizers, since the polarization vectors can be described with simple Cartesian coordinates independent from the orientation of the polarizer surface.
When the two polarization states are relative to the direction of a surface, they are termed s and p. This distinction between Cartesian and s–p polarization can be negligible in many cases, but it becomes significant for achieving high contrast and with wide angular spreads of the incident light. Certain crystals, due to the effects described by crystal optics, show dichroism, preferential absorption of light, polarized in particular directions, they can therefore be used as linear polarizers. The best known crystal of this type is tourmaline. However, this crystal is used as a polarizer, since the dichroic effect is wavelength dependent and the crystal appears coloured. Herapathite is dichroic, is not coloured, but is difficult to grow in large crystals. A Polaroid polarizing filter functions on an atomic scale to the wire-grid polarizer, it was made of microscopic herapathite crystals. Its current H-sheet form is made from polyvinyl alcohol plastic with an iodine doping. Stretching of the sheet during manufacture causes the PVA chains to align in one particular direction.
Valence electrons from the iodine dopant are able to move linearly along the polymer chains, but not transverse to them. So incident light polarized; the durability and practicality of Polaroid makes it the most common type of polarizer in use, for example for sunglasses, photographic filters, liquid crystal displays. It is much cheaper than other types of polarizer. A modern type of absorptive polarizer is made of elongated silver nano-particles embedded in thin glass plates; these polarizers are more durable, can polarize light much better than plastic Polaroid film, achieving polarization ratios as high as 100,000:1 and absorption of polarized light as low as 1.5%. Such glass polarizers perform best for short-wavelength infrared light, are used in optical fiber communications. Beam-splitting polarizers split the incident beam into two beams of differing linear polarization. For an ideal polarizing beamsplitter these would be polarized, with orthogonal polarizations. For many common beam-splitting polarizers, only one of the two output beams is polarized.
The other contains a mixture of polarization states. Unlike absorptive polarizers, beam splitting polarizers do not need to absorb and dissipate the energy of the rejected polarization state, so they are more suitable for use with high intensity beams such as laser light. True polarizing beamsplitters are useful where the two polarization components are to be analyzed or used simultaneously; when light reflects at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Light polarized in the plane is said to be p-polarized, while that polarized perpendicular to it is s-polarized. At a special angle known as Brewster's angle, no p-polarized light is reflected from the surface, thus all reflected light must be s-polarized, with an electric field perpendicular to the plane of incidence. A simple linear polarizer can be made by tilting a stack of glass plates at Brewster's angle to the beam.
Some of the s-polarized light is reflected from each surface of each plate. For a stack of plates, each reflection depletes the incident beam of s-polarized light, leaving a greater fraction of p-polarized light in the transmitted beam at each stage. For visible light in air and typical glass, Brewster's angle is about 57°, about 16% of the s-polarized light present in the beam is reflected for each air-to-glass or glass-to-air transition, it takes many plates to achieve mediocre polarization of the transmitted beam with this approach. For a stack of 10 plates, about 3% of the s-polarized light is transmitted; the reflected beam, while polarized, is spread out and may not be useful. A more useful polarized beam can be obtained by tilting the pile of plates at a steeper angle to the incident beam. Counterintuitively, using incident angles greater than Brewster's angle yields a higher degree of polarization of the transmitted beam, at the expense of decreased overall transmission. For angles of incidence steeper than 80° the polarization of the transmitted beam can approach 100% with as few as four plates, although the transmitted intensity is low in this case.
Adding more plates and reducing
Polarizing filter (photography)
A polarizing filter or polarising filter is placed in front of the camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections tend to be at least linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph; the rotational orientation of the filter is adjusted for the preferred artistic effect. For modern cameras, a circular polarizer is used; this additional step avoids problems with auto-focus and light-metering sensors within some cameras, which otherwise may not function reliably with a simple linear polariser. Light reflected from a non-metallic surface becomes polarized. A polarizer rotated to pass only light polarized in the direction perpendicular to the reflected light will absorb much of it; this absorption allows glare reflected from, for example, a body of a road to be reduced. Reflections from shiny surfaces are reduced; this allows the natural detail of what is beneath to come through.
Reflections from a window into a dark interior can be much reduced. Some of the light coming from the sky is polarized; the electrons in the air molecules cause a scattering of sunlight in all directions. This explains, but when looked at from the sides, the light emitted from a specific electron is polarized. Hence, a picture taken in a direction at 90 degrees from the sun can take advantage of this polarization; the effect is visible in a band of 15° to 30° measured from the optimal direction. Use of a polarizing filter, in the correct direction, will filter out the polarized component of skylight, darkening the sky. Perpendicularly incident light waves tend to reduce clarity and saturation of certain colors, which increases haziness; the polarizing lens absorbs these light waves, rendering outdoor scenes crisper with deeper color tones in subject matter such as blue skies, bodies of water and foliage. Much light is differentiated by polarization, e.g. light passing through crystals like sunstones or water droplets producing rainbows.
The polarization of the rainbow is caused by the internal reflection. The rays strike the back surface of the drop close to the Brewster angle. Polarizing filters can be rotated to minimise admission of polarised light, they are mounted in a rotating collar for this purpose. Rotating the polarizing filter will make rainbows and other polarized light stand out or nearly disappear depending on how much of the light is polarized and the angle of polarization; the benefits of polarizing filters are the same in digital or film photography. While software post-processing can simulate many other types of filter, a photograph does not record the light polarization, so the effects of controlling polarization at the time of exposure cannot be replicated in software. There are two types of polarizing filters available, linear and "circular", which have the same effect photographically, but the metering and auto-focus sensors in certain cameras, including all auto-focus SLRs, will not work properly with linear polarizers because the beam splitters used to split off the light for focusing and metering are polarization-dependent.
Linearly-polarized light may defeat the action of the Anti-aliasing filter on the imaging sensor. "Circular" polarizing photographic filters consist of a linear polarizer on the front, with a quarter-wave plate on the back. The quarter-wave plate converts the selected polarization to circularly polarized light inside the camera; this works with all types of cameras, because mirrors and beam-splitters split circularly polarized light the same way they split unpolarized light. Linear polarizing filters can be distinguished from circular polarizers. In linear polarizing filters, the polarizing effect works regardless of which side of the filter the scene is viewed from. In "circular" polarizing filters, the polarizing effect works when the scene is viewed from the male threaded side of the filter, but does not work when looking through it backwards. Polarizing filters reduce the light passed through to the film or sensor by about one to three stops depending on how much of the light is polarized at the filter angle selected.
Auto-exposure cameras will adjust for this by widening the aperture, lengthening the shutter, and/or increasing the ASA/ISO speed of the camera. Polarizing filters can be used deliberately to reduce available light and allow use of wider apertures to shorten depth of field for certain focus effects; some companies make adjustable neutral density filters by having two linear polarising layers. When they are at 90° to each other, they let zero light in, admitting more as the angle decreases. Circular polarizers Polarizer Polarized 3D glasses Polarizer another must-have filter
John William Strutt, 3rd Baron Rayleigh
John William Strutt, 3rd Baron Rayleigh, was a British scientist who made extensive contributions to both theoretical and experimental physics. He spent all of his academic career at the University of Cambridge. Among many honours, he received the 1904 Nobel Prize in Physics "for his investigations of the densities of the most important gases and for his discovery of argon in connection with these studies." He served as President of the Royal Society from 1905 to 1908 and as Chancellor of the University of Cambridge from 1908 to 1919. Rayleigh provided the first theoretical treatment of the elastic scattering of light by particles much smaller than the light's wavelength, a phenomenon now known as "Rayleigh scattering", which notably explains why the sky is blue, he studied and described transverse surface waves in solids, now known as "Rayleigh waves". He contributed extensively to fluid dynamics, with concepts such as the Rayleigh number, Rayleigh flow, the Rayleigh–Taylor instability, Rayleigh's criterion for the stability of Taylor–Couette flow.
He formulated the circulation theory of aerodynamic lift. In optics, Rayleigh proposed a well known criterion for angular resolution, his derivation of the Rayleigh–Jeans law for classical black-body radiation played an important role in birth of quantum mechanics. Rayleigh's textbook The Theory of Sound is still used today by engineers. Strutt was born on 12 November 1842 at Langford Grove in Essex. In his early years he suffered from poor health, he attended Eton College and Harrow School, before going on to the University of Cambridge in 1861 where he studied mathematics at Trinity College, Cambridge. He obtained a Bachelor of Arts degree in 1865, a Master of Arts in 1868, he was subsequently elected to a Fellowship of Trinity. He held the post until his marriage to Evelyn Balfour, daughter of James Maitland Balfour, in 1871, he had three sons with her. In 1873, on the death of his father, John Strutt, 2nd Baron Rayleigh, he inherited the Barony of Rayleigh, he was the second Cavendish Professor of Physics at the University of Cambridge, from 1879 to 1884.
He first described dynamic soaring in the British journal Nature. From 1887 to 1905 he was Professor of Natural Philosophy at the Royal Institution. Around the year 1900 Rayleigh developed the duplex theory of human sound localisation using two binaural cues, interaural phase difference and interaural level difference; the theory posits that we use two primary cues for sound lateralisation, using the difference in the phases of sinusoidal components of the sound and the difference in amplitude between the two ears. In 1919, Rayleigh served as President of the Society for Psychical Research; as an advocate that simplicity and theory be part of the scientific method, Rayleigh argued for the principle of similitude. Rayleigh was elected Fellow of the Royal Society on 12 June 1873, served as president of the Royal Society from 1905 to 1908. From time to time Rayleigh participated in the House of Lords, he died on 30 June 1919, in Essex. He was succeeded, as the 4th Lord Rayleigh, by his son Robert John Strutt, another well-known physicist.
Lord Rayleigh was buried in the graveyard of All Saints' Church in Terling in Essex. The rayl unit of acoustic impedance is named after him. Rayleigh was an Anglican. Though he did not write about the relationship of science and religion, he retained a personal interest in spiritual matters; when his scientific papers were to be published in a collection by the Cambridge University Press, Strutt wanted to include a religious quotation from the Bible, but he was discouraged from doing so, as he reported: When I was bringing out my Scientific Papers I proposed a motto from the Psalms, "The Works of the Lord are great, sought out of all them that have pleasure therein." The Secretary to the Press suggested with many apologies that the reader might suppose that I was the Lord. Still, he had his wish and the quotation was printed in the five-volume collection of scientific papers. In a letter to a family member, he wrote about his rejection of materialism and spoke of Jesus Christ as a moral teacher: I have never thought the materialist view possible, I look to a power beyond what we see, to a life in which we may at least hope to take part.
What is more, I think that Christ and indeed other spiritually gifted men see further and truer than I do, I wish to follow them as far as I can. He was an early member of the Society for Psychical Research, he remained open to the possibility of supernatural phenomena. Rayleigh was the president of the SPR in 1919, he gave a presidential address in the year of his death but did not come to any definite conclusions. The lunar crater Rayleigh as well as the Martian crater Rayleigh were named in his honour; the asteroid 22740 Rayleigh was named after him on 1 June 2007. A type of surface waves are known as Rayleigh waves; the rayl, a unit of specific acoustic impedance, is named for him. Rayleigh was awarded with: Smith's Prize Royal Medal Matteucci Medal Member of the Royal Swedish Academy of Sciences Copley Medal Nobel Prize for Physics Elliott Cresson Medal Rumford Medal Lord Rayleigh was among the original recipients of the O
Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither lit nor dark; the word twilight is used to denote the periods of time when this illumination occurs. The farther the Sun is below the dimmer the twilight; when the Sun reaches 18° below the horizon, the twilight's brightness is nearly zero, evening twilight becomes nighttime. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight. Owing to its distinctive quality the absence of shadows and the appearance of objects silhouetted against the lit sky, twilight has long been popular with photographers, who sometimes refer to it as "sweet light", painters, who refer to it as the blue hour, after the French expression l'heure bleue. Twilight should not be confused with auroras, which can have a similar appearance in the night sky at high latitudes.
By analogy with evening twilight, the word twilight is sometimes used metaphorically, to imply that something is losing strength and approaching its end. For example old people may be said to be "in the twilight of their lives"; the collateral adjective for twilight is crepuscular, which may be used to describe the behavior of insects and mammals that are most active during this period. Twilight is defined according to the solar elevation angle θs, the position of the geometric center of the sun relative to the horizon. There are three established and accepted subcategories of twilight: civil twilight, nautical twilight, astronomical twilight. Three subcategories of twilight are established and accepted: civil twilight, nautical twilight, astronomical twilight. Morning civil twilight begins when the geometric center of the sun is 6° below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon. In the United States' military, the initialisms BMCT and EECT are used to refer to the start of morning civil twilight and the end of evening civil twilight, respectively.
Civil dawn is preceded by morning nautical twilight and civil dusk is followed by evening nautical twilight. Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to distinguish terrestrial objects. Enough illumination renders artificial sources unnecessary for most outdoor activities. At civil dawn and at civil dusk sunlight defines the horizon while the brightest stars and planets can appear; as observed from the Earth, sky-gazers know Venus, the brightest planet, as the "morning star" or "evening star” because they can see it during civil twilight. Lawmakers have enshrined the concept of civil twilight; such statutes use a fixed period after sunset or before sunrise, rather than how many degrees the sun is below the horizon. Examples include the following periods:; the period may affect when extra equipments, such as anti-collision lights, are required for aircraft to operate. In the US, civil twilight for aviation is defined in Part 1.1 of the Federal Aviation Regulations as the time listed in the American Air Almanac.
Morning nautical twilight begins when the geometric center of the sun is 12 degrees below the horizon in the morning and ends when the geometric center of the sun is 6 degrees below the horizon in the morning. Evening nautical twilight begins when the geometric center of the sun is 6 degrees below the horizon in the evening and ends when the geometric center of the sun is 12 degrees below the horizon in the evening. Nautical dawn is the moment when the geometric center of the Sun is 12 degrees below the horizon in the morning, it is followed by morning nautical twilight. Nautical dusk is the moment when the geometric center of the Sun is 12 degrees below the horizon in the evening, it marks the end of evening nautical twilight. Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea as they cannot see the horizon. At nautical dawn and nautical dusk, the human eye finds it difficult, if not impossible, to discern traces of illumination near the sunset or sunrise point of the horizon.
Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference. Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations. Nautical twilight has military considerations as well; the initialisms BMNT and EENT are considered when planning military operations. A military un
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths; when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as n = c v, where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times as fast in vacuum as in water. The refractive index determines how much the path of light is bent, or refracted, when entering a material; this is described by Snell's law of refraction, n1 sinθ1 = n2 sinθ2, where θ1 and θ2 are the angles of incidence and refraction of a ray crossing the interface between two media with refractive indices n1 and n2. The refractive indices determine the amount of light, reflected when reaching the interface, as well as the critical angle for total internal reflection and Brewster's angle; the refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is v = c/n, the wavelength in that medium is λ = λ0/n, where λ0 is the wavelength of that light in vacuum.
This implies that vacuum has a refractive index of 1, that the frequency of the wave is not affected by the refractive index. As a result, the energy of the photon, therefore the perceived color of the refracted light to a human eye which depends on photon energy, is not affected by the refraction or the refractive index of the medium. While the refractive index affects wavelength, it depends on photon frequency and energy so the resulting difference in the bending angle causes white light to split into its constituent colors; this is called dispersion. It can be observed in prisms and rainbows, chromatic aberration in lenses. Light propagation in absorbing materials can be described using a complex-valued refractive index; the imaginary part handles the attenuation, while the real part accounts for refraction. The concept of refractive index applies within the full electromagnetic spectrum, from X-rays to radio waves, it can be applied to wave phenomena such as sound. In this case the speed of sound is used instead of that of light, a reference medium other than vacuum must be chosen.
The refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 299792458 m/s, the phase velocity v of light in the medium, n = c v. The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves; the definition above is sometimes referred to as the absolute refractive index or the absolute index of refraction to distinguish it from definitions where the speed of light in other reference media than vacuum is used. Air at a standardized pressure and temperature has been common as a reference medium. Thomas Young was the person who first used, invented, the name "index of refraction", in 1807. At the same time he changed this value of refractive power into a single number, instead of the traditional ratio of two numbers; the ratio had the disadvantage of different appearances. Newton, who called it the "proportion of the sines of incidence and refraction", wrote it as a ratio of two numbers, like "529 to 396".
Hauksbee, who called it the "ratio of refraction", wrote it as a ratio with a fixed numerator, like "10000 to 7451.9". Hutton wrote it as a ratio with a fixed denominator, like 1.3358 to 1. Young did not use a symbol for the index of refraction, in 1807. In the next years, others started using different symbols: n, m, µ; the symbol n prevailed. For visible light most transparent media have refractive indices between 1 and 2. A few examples are given in the adjacent table; these values are measured at the yellow doublet D-line of sodium, with a wavelength of 589 nanometers, as is conventionally done. Gases at atmospheric pressure have refractive indices close to 1 because of their low density. All solids and liquids have refractive indices above 1.3, with aerogel as the clear exception. Aerogel is a low density solid that can be produced with refractive index in the range from 1.002 to 1.265. Moissanite lies at the other end of the range with a refractive index as high as 2.65. Most plastics have refractive indices in the range from 1.3 to 1.7, but some high-refractive-index polymers can have values as high as 1.76.
For infrared light refractive indices can be higher. Germanium is transparent in the wavelength region from 2 to 14 µm and has a refractive index of about 4. A type of new materials, called topological insulator, was found holding higher refractive index of up to 6 in near to mid infrared frequency range. Moreover, topological insulator material are transparent; these excellent properties make them a type of significant materials for infrared optics. According to the theory of relativity, no information can travel faster than the speed of light in vacuum, but this does not mean that the refractive index cannot be lower than 1; the refractive index measures the phase velocity of light. The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, thereby give a refractive index below 1; this can occur close to resonance frequencies, for absorbing media, in plasmas, for X-rays. In the X-ray regime the refractive indices are